Browsing by Department "Ecology, Evolution and Behavior"
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Item A comparative study of anuran chromosomes(1933) Hayes, Edwin Spencer; Painter, Theophilus S. (Theophilus Shickel), 1889-1969Item A comparative study of brackish water fishes(1950) Kemp, Robert J.; Not availableItem A report on four rare plant species of Big Bend National Park, Texas(2017-12) Schmidt, Olivia Grace; Fowler, Norma L.Big Bend National Park in Texas serves as a refuge for many rare plant species, many of which can be found nowhere else in the United States. This study is a summary and synthesis of the available research that has been done on four rare plants in Big Bend National Park: Coryphantha ramillosa subspecies ramillosa (bunched cory cactus), Echinocereus chisoensis var. chisoensis (Chisos Mountain hedgehog cactus), Echinomastus mariposensis (Lloyd’s mariposa cactus), and Festuca ligulata (Guadalupe fescue). The objectives of this study were (1) to review and synthesize all existing published and unpublished studies of these species, with a focus on known threats, conservation priorities, and research needs, (2) to create a GIS database of available public and private data relevant to these species, and (3) using this database, to conduct a preliminary analysis identify the primary habitat characteristics of each of these species at both local and landscape scales. At the landscape scale, geological substrate, elevation, and topographic position characterized species' habitats. At a local scale, slope and sometimes soil unit determined species presence. Further research is needed on each species. Each of these species faces multiple threats, and collaborations between government agencies, private conservation organizations, private landowners, and researchers may be essential to the recovery of these species.Item A study of the role of cortisol in the osmoregulatory behavior of a teleost, Mugil cephalus(1973) Jackson, Peter Richard, 1948-; Not availableThe effects of injections of metyrapone, an inhibitor of corticosteroid synthesis, cortisol and a combination of these treatments were studied to determine the effects of corticosteroids on the plasma sodium, potassium and calcium levels in marine and fresh water mullet, Mugil cephalus. The metyrapone effects could not be interpreted in terms of the physiology of the teleost interrenal gland and were probably due to the narcotic-like side effects of the drug. Cortisol was found to enhance the osmoregulatory mechanisms necessary for the survival of the fish in sea and fresh water. Plasma sodium, potassium and calcium levels were significantly lowered in sea water mullet after cortisol injections, indicating that this hormone is effective in enhancing the normal elimination of excess ions in this medium. Fresh water mullet were able to accumulate significantly higher levels of sodium, potassium and calcium after being injected with cortisol, indicating that the hormone enhances their ability to osmoregulate in a hypotonic environment. A gas chromatographic analysis of cortisol extracted from the plasma of mullet kept in sea and fresh water for 24 hours indicated that fresh water fish had about twice the circulating levels of this hormone. It was concluded that the corticosteroids are necessary for the maintenance of ions in sea and fresh water mullet and that the fish require more of these hormones to survive in fresh waterItem A study of the thyroid-pituitary relationship in an elasmobranch, Dasyatis sabina(1973) Jackson, Rodney George, 1942-; Not availableThe pituitary-thyroid relationship has been studied in vitro in the stingray, Dasyatis sabina. The addition of mammalian TSH failed to stimulate incubated stingray thyroid glands, but mammalian TSH did stimulate teleost thyroids in parallel experiments. Homogenates of whole D. sabina pituitary significantly increased the release of thyroxine from the stingray thyroid, while homogenates of whole teleost pituitary had no effect. The greatest thyroid stimulating activity was found in the ventral lobe of the pars distalis. No seasonal or sexual differences were observed in the relationship between thyroid weight and animal weight. The ratio of total iodine to thyroxine iodine did not vary between sexes or between stimulated and unstimulated glands in vitro. It was concluded that the thyroid is associated with reproduction because (1) thyrotropic activity was located in the region known to contain gonadotrophic activity, and (2) the spontaneous rate of thyroxine release from incubated thyroid glands increased in females at the time of sexual maturation (release rates of juvenile males, adult males, and adult females were similar). There were no changes in the spontaneous rates of thyroxine release that could be correlated with seasonal changes in environmental salinity or temperature or with migratory movementsItem Aggression, space use, and dominance in a social cichlid fish(2021-12-03) Friesen, Caitlin Nicole; Hofmann, Hans (Hans A.); Ryan, Michael J. (Michael Joseph), 1953-; Alonzo, Suzanne; Gore, Andrea C.; Phelps, Steven M.How do individuals adjust their behavior across changing contexts to obtain resources needed to survive and reproduce? In group living animals, behavioral variation is regulated by the surrounding group and the underlying patterns of hormones and neural gene expression. We can often learn more by studying an individual in response to different challenging situations than in an undisturbed setting. For example, the challenge hypothesis has provided a useful framework across taxa to understand causal relationships between androgen hormones and aggressive behavior in response to reproductively motivated social challenges (reviewed in Chapter 1). In contrast, we still have much to learn about dynamic interactions across molecular, physiological, morphological, or behavioral traits that may be functionally linked (the “integrated phenotype”), or the effects of an individual response on others within the social group. There is a paucity of data focused on acute measures of dynamic space use, social behavior, and hormone levels in response to a challenge. To address this gap in our understanding, I conducted a series of experiments using the African cichlid fish, Astatotilapia burtoni. This species occurs in dynamic social groups, and males form dominance hierarchies where socially dominant males defend territories to gain exclusive access to reproductive opportunities. In chapter 2, I examined behavioral variation across dominant males in relation to space use, hormone levels, and gene expression in response to a perturbation to create an integrative metric of territorial behavior. My results show that an integrative analysis of behavioral variation can identify sub-types of dominant male and predict their response to perturbation. In chapter 3, I utilized this integrative metric to investigate how resident dominant males respond to a male intruder within the social group. I found that resident males exhibited distinct types of aggressive displays that differed depending on the outcome of intrusion. Finally, in chapter 4, I investigated whether traits involved in territorial behavior exhibited patterns of covariation by hormonally manipulating one dominant male within each social group. I found context-dependent associations between hormones, space use, and social behavior in dominant males. Taken together, my research highlights the importance of considering acute, dynamic measures of social behavior and space use along with physiological and neural mechanisms involved in individual responses to challenges within naturalistic social groups.Item Analyses of infectious disease data with attention to heterogeneity(2013-08) O'Dea, Eamon Brendin; Wilke, C. (Claus); Meyers, Lauren AncelThis work comprises three projects that extend previous models to include features of practical significance for the statistical analysis of infectious disease data. In the first, we find from a simulation study how the degree of heterogeneity in the number contacts that individuals have affects the relationship between estimates of a pathogen's effective population size based on coalescent theory and the true prevalence and incidence of that pathogen. In the second, we find that aggregating data from many small outbreaks allows the parameters of stochastic epidemic models to be consistently estimated with a generalized linear model. Application of this method to a set of 77 small norovirus outbreaks reveals interesting differences in the transmission parameters between hospital and nursing-home outbreaks. In the third project, we gain insight into HIV contact networks in the United States by fitting data from a number of surveys to a simple stochastic model of a dynamic network.Item An analysis of the effect of varying the duration of X-ray treatment upon the frequency of mutations(1931) Oliver, Clarence P. (Clarence Paul), 1898-; Not availableItem Androgen modulation of vocal circuits in Alston’s singing mice, scotinomys teguina(2020-12-05) Zheng, Da-Jiang; Phelps, Steven Michael, 1970-; Gore, Andrea; Ryan, Mike; Zakon, HaroldSexual dimorphism in brain and behavior are often among the most conspicuous forms of intraspecific variation. Gonadal state often shape differences between the sexes. Steroids may directly lead to changes in most of levels of biological organization. In the vertebrate brain for example, expression of steroid receptors and its ligands may lead to direct consequences from the motor systems to the limbic system. The songs of Alston’s singing mouse (Scotinomys teguina) are sexually dimorphic and are composed of a series of rapidly repeated, broad-bandith, frequency-modulated notes. Males sing more often than females. Castration makes songs softer, shorter, and less common. This decrease in performance can be rescued by the application of dihydrotestosterone suggesting that androgen receptors play a critical role. In this dissertation, I seek to further understand the role of the androgen receptor in influencing the singing behavior of this species by three interconnected aims. First, we map the sites of androgen sensitivity along a pathway that spans from limbic structures to vocal control regions and quantify differences in nuclear androgen receptor-like labeling in male and female brains. Second, we use a transsynaptic virus to examine how the forebrain connects to the motor control of the jaw and larynx. Third, we manipulate the hormonal milieu by castrating and then implanting DHT to observe changes in both brain and behavior. Together we gained better insight into not only how androgen receptors mediate changes to vocalization but also how gonadal state lead to variation between the sexes.Item Ant genotype, not fungus genotype, predicts aggression in the asexual fungus-farming ant, Mycocepurus smithii(2015-08) Barrett, Brian Timothy; Kirkpatrick, Mark, 1956-; Mueller, Ulrich GHamilton's rule specifies that the relatedness of two individuals will have a large bearing on whether an altruistic action is performed; however, it says little with regards to whether individuals are able to discern that relatedness. In this study we examine whether the fungus-farming ant Mycocepurus smithii uses genotypic information to decide whether to attack an introduced queen or if it utilizes environmental cues from the fungus that they cultivate. We performed 180 blind trials in which we introduced queens to queenless mesocosms and recorded and scored aggression behavior directed towards the queen. We find strong evidence that M. smithii uses genotype to mediate aggression, but find no support that fungal cultivar plays any role in recognition. These results serve to support Hamilton's notion that relatedness acts as a gateway to altruism.Item Applying mathematical and statistical methods to the investigation of complex biological questions(2013-08) Scarpino, Samuel Vincent; Kirkpatrick, Mark, 1956-; Meyers, Lauren AncelThe research presented in this dissertation integrates data and theory to examine three important topics in biology. In the first chapter, I investigate genetic variation at two loci involved in a genetic incompatibility in the genus Xiphophorus. In this genus, hybrids develop a fatal melanoma due to the interaction of an oncogene and its repressor. Using the genetic variation data from each locus, I fit evolutionary models to test for coevolution between the oncogene and the repressor. The results of this study suggest that the evolutionary trajectory of a microsatellite element in the proximal promoter of the repressor locus is affected by the presence of the oncogene. This study significantly advances our understanding of how loci involved in both a genetic incompatibility and a genetically determined cancer evolve. Chapter two addresses the role polyploidy, or whole genome duplication, has played in generating flowering plant diversity. The question of whether polyploidy events facilitate diversification has received considerable attention among plant and evolutionary biologists. To address this question, I estimated the speciation and genome duplication rates for 60 genera of flowering plants. The results suggest that diploids, as opposed to polyploids, generate more species diversity. This study represents the broadest comparative analysis to date of the effect of polyploidy on flowering plant diversity. In the final chapter, I develop a computational method for designing disease surveillance networks. The method is a data-driven, geographic optimization of surveillance sites. Networks constructed using this method are predicted to significantly outperform existing networks, in terms of information quality, efficiency, and robustness. This work involved the coordinated efforts of researchers in biology, epidemiology, and operations research with public health decision makers. Together, the results of this dissertation demonstrate the utility of applying quantitative theory and statistical methods to data in order to address complex, biological processes.Item Bee conservation and epidemiology across scales(2023-04-19) Ivers, Nicholas Alexander; Jha, Shalene; Gilbert, Lawrence; Juenger, Thomas; Muth, Felicity; Brown, MarkBees provide essential pollination services that support floral biodiversity and maintain global agricultural production. Bees also face numerous stressors and challenges to their conservation, including land-use change and increasing pressure from detrimental parasites and pathogens. Heavily modified landscapes often lack the nesting and/or foraging resources necessary to support a diverse community of bees, therefore anthropogenic land use change poses a major risk to the diversity and abundance of bee communities, and the health and long-term stability of their populations. Land use change alters the distribution and composition of plant and pollinator communities and their mutualistic interactions with plants. These interactions are especially critical given that many bee parasites are transmitted indirectly through shared use of flowers, as well as among nestmates of social bees. To better understand the stressors and conservation priorities for bees, I explore the drivers of parasite prevalence 1) at the landscape scale by surveying bumble bees across a 1000km gradient, 2) at regional scales by examining drivers of parasitism within urban gardens, and 3) at the local scale by studying shared parasitism across bee communities. As central place foragers, bees are tied to the local landscape, making them susceptible to land use and climate change, which alters the suitability and accessibility of local habitats. This has long-term consequences for gene flow, population structure, and genetic variation, with major consequences for parasitism. I incorporate spatially explicit environmental, biogeographic, and land-use data in combination with genetically derived host population data to conduct a large-scale epidemiological assessment of the drivers of parasite prevalence, finding that spatial patterns of parasitism are isolated by the same factors isolating host populations. At regional scales bee community composition varies greatly from patch to patch depending on nesting and foraging resource availability. The quality of resources and accessibility of local habitats determines the abundance and diversity of bees with direct impacts pathogen transmission. By studying bumble bees in urban gardens, I found that nest site limitations lead to higher parasite prevalence in local bumblebees, while certain gardens support more diverse bee communities which have lower pathogen pressure as a result. Lastly, at local scales, parasite prevalence varies greatly between species in the community, yet there is much we do not understand about parasite transmission across communities and whether parasite host ranges are related to functional differences in bee traits or foraging behavior. I use plant-pollinator networks to characterize bee foraging behavior, and map potential routes of interspecific parasite transmission through diet overlap. There is great potential for parasite sharing in these networks, with diet overlap and foraging specialization being key factors in addition to functional differences in species’ traits and life history. Overall, I provide evidence for many drivers of parasite prevalence across spatial and conceptual scales with impacts for conservation planning and landscape epidemiology.Item Behavioral and molecular mechanisms of pheromone transmission in the honey bee (Apis mellifera)(2017-08) Ma, Rong, Ph. D.; Mueller, Ulrich G.; Hofmann, Johann; Gilbert, Lawrence E; Jha, Shalene; Grozinger, Christina MThe European honey bee (Apis mellifera) has a sophisticated system of pheromonal signals that mediate a wide range of behaviors important for their fitness, including reproductive dominance, nest defense, and cooperative brood care. In honey bees, there are two distinct pheromones emitted by larvae, brood pheromone and (E)-beta-ocimene. By integrating behavior, chemical ecology, and transcriptomics, this dissertation analyzes several key stages in signal transmission in a systematic effort to understand how these two pheromones affect behavior, and in the process, generates a synthetic understanding of a highly complex system of communication. Previous studies have explored behavioral and gene expression patterns related to honey bee pheromones; however, none have compared the roles that two divergent pheromones from a common source play in rapid regulation of foraging behavior. Furthermore, while previous studies have investigated the mechanisms of pheromone detection and the factors involved in regulation of foraging behavior, it remains unclear how individual responses to pheromone exposure scales to colony-level changes in behavior. By investigating the behavioral, physiological, and genomic influences of honey bee chemical communication, this dissertation links phenotypic plasticity in behavior to gene expression profiles in the brain and provides insights into the evolution of a sophisticated chemical language.Item Biotic and abiotic controls on carbon dynamics in a Central Texas encroaching savanna(2014-12) Thijs, Ann; Hawkes, Christine V.; Litvak, Marcy E.Anthropogenic activities are responsible for increases in atmospheric CO₂ and climate change. These increases are partly counterbalanced by natural processes, such as carbon uptake in land surfaces. These processes are themselves subject to climate change, creating a coupled carbon-climate system. I investigated the carbon sink that woody encroachment represents, using a Central Texas savanna as study site, and studied how climatic factors influence this carbon sink. Woody plant encroachment, a worldwide structural change in grassland and savanna ecosystems, alters many ecosystem properties, but the net effect on the carbon balance is uncertain. Woody encroachment represents one of the key uncertainties in the US carbon balance, and demands a more detailed understanding. To come to a process-based understanding of the encroachment effect on carbon dynamics, I analyzed patterns of carbon exchange using eddy-covariance technology. I expected the imbalance between carbon uptake and release processes associated with the encroaching trees specifically, to be responsible for the carbon sink. I also expected that the sink would vary in time, due to strong links between carbon fluxes and soil water in this semi-arid ecosystem. I further studied the ecophysiology of the dominant species, as well as soil respiration processes under different vegetation types, and scaled these findings in space and time. I found that the ecosystem was a significant carbon sink of 405 g C m⁻² yr⁻¹. The encroaching trees increased photosynthesis by 180% and decreased soil respiration by 14%, compared to the grassland, resulting in a strong carbon sink due to the encroachment process. The encroaching process also altered carbon dynamics in relation to climatic drivers. The evergreen species Ashe juniper effectively lengthened the growing season and widened the temperature range over which the ecosystem acts as a carbon sink. The drought resistance of the encroaching trees reduced the sensitivity of this savanna to drought. I conclude that encroachment in Central Texas savannas increased the carbon sink strength by increasing the carbon inputs into the ecosystem. Woody encroachment also reduced the sensitivity to climatic drivers. These two effects constitute a direct effect, as well as a negative feedback to the coupled carbon-climate system.Item Brazilian central Cerrado lizards in introduced Eucalyptus plantations : human mediated habitat disturbance effects from community diversity to population divergence(2012-05) Gainsbury, Alison Melissa; Pianka, Eric R.; Hillis, David M.; Bell, Christopher J.; Simpson, Beryl B.; Bolnick, Daniel I.Approximately two thirds of the world’s land is directly supporting human population contributing to an accumulation of disturbed habitats. This dissertation investigates the impact of human mediated habitat disturbance, in introduced Eucalyptus plantations, on community diversity and population divergence using Brazilian Cerrado lizards as a model. Data was collected along a gradient from undisturbed cerrado to disturbed Eucalyptus plantations. Community diversity differences and indicator species were identified. Furthermore, the role of phenotypic divergences were determined based on populations able to persist in disturbed habitats. Dispersal, food availability (body condition), competition and predation (caudal autotomy) were tested as potential mechanisms driving phenotypic divergences. Additionally, I investigated phylogenetic community structure differences between habitats to test for a phylogenetic signal to disturbance. The evidence showed community diversity indices were significantly lower in Eucalyptus plantations with a decrease along the cerrado– Eucalyptus gradient. Furthermore, 29 % of the Cerrado species suffered local extinctions in the disturbed habitat and of these 80 % are endemic species. One indicator species was identified for the disturbed habitat and seven species were identified for the undisturbed habitat. Species able to persist in both habitats demonstrated morphological trait divergences. These species showed short dispersal distances with only two individuals dispersing between habitats indicating a mechanism driving the observed phenotypic divergences. Another mechanism is body condition, which was higher in the disturbed habitats, reflecting increased food availability possibly due to the decreased abundances. Caudal autotomy showed no difference between the habitats indicating that competition and predation are not driving phenotypic divergences. Phylogenetic community structure demonstrated a phylogenetic signal to disturbance. The undisturbed habitat consists of communities with more closely related species compared to the disturbed habitat: indicating evolutionary forces such as habitat filtering as the stronger process structuring these communities. Whereas, disturbed communities are structured by ecological forces such as competition. This research provides information for the preservation and maintenance of the Cerrado biodiversity and has an even broader impact since habitat change caused by human activities touches a plethora of ecosystems.Item Capturing the social memory of librarianship(2013-05) Smith, Alan Arro; Roy, LorieneThis research has identified elements of the social memory of librarianship from the last half of the twentieth century by collecting and examining thirty-four oral history interviews of librarians at the end of their careers. These professional life stories trace an important arc through the history of library and information science. Many of these librarians began their careers prior to the use of any form of computer technology in libraries. This cohort ushered in a wave of technological innovations that has revolutionized the access to information. These oral history interviews are part of the Capturing Our Stories Oral History Program of Retiring/Retired Librarians sponsored by the American Library Association and the School of Information at the University of Texas. The social memory includes regret and nostalgia for the librarianship practiced at the beginning of their careers, excitement and wonder about how technology has fundamentally changed the profession, and perspectives on the popular stereotype associated with their careers.Item Causes and consequences of color polymorphism in Rambur’s forktail (Ischnura ramburii)(2013-05) Gering, Eben Jordan; Cummings, Molly E.; Ryan, Michael J; Kirkpatrick, Mark A; Bolnick, Daniel I; Abbott, John CVariation in male and female forms occurs in countless animal taxa, and has fascinated evolutionary biologists since Darwin and Wallace. The underpinnings of male variation have been elucidated in diverse groups; less is known about the selective forces that diversify female forms in nature. Female-polymorphic damselflies provide ideal systems in which to study how female variation evolves. Color polymorphic damselflies typically contain one female morph that resembles the male (“andromorph”) and one or more alternative morphs with distinctive coloration (“gynomorphs” or “heteromorphs”). My thesis draws upon the unique context of a biological invasion to elucidate factors that promote and maintain this variation in female color. Empirical work in my dissertation is focused upon Rambur’s Forktail (Ischnura ramburii), a species native to the Americas that invaded Hawaii in the 1970s. I first examine whether female color morphs diverge in mating rates or other reproductive traits within the native and invasive range, to see whether such traits might affect morph frequency dynamics in the invasion context (Chapter 2). Next, I test whether variation in selective regimes, both across female development and among populations, predicts variation in andromorph coloration (Chapter 3). Upon finding andromorphs to follow predictions of mimicry theory, I ask whether andromorph presence might result in increased male-male interaction rates, due to sex recognition errors (Chapter 4). Finally, I document recent, rapid evolution of andromorphy within Hawaii populations, and conduct mesocosm experiments to test the potential for density- and frequency-dependent selection to promote and maintain color polymorphism. Results indicate 1) andromorphs may benefit from reduced mating, but male-like morphology may also incur reproductive constraints; 2) andromorph color variation accords with mimicry theory: andromorphs resemble syntopic males, and resemblance is maximized after reproductive onset; 3) male-male interactions increase in the presence of andromorphs, to male detriment; 4) gynomorphs are subject to negative-frequencydependence in high-density populations, which may have driven the rapid evolution of andromorphy in Hawaii following introduction to the islands. These findings offer new insights into multiple mechanisms by which color polymorphism can arise and be maintained within native and invasive contexts.Item Causes and consequences of movement : the interaction between foraging and landscape patterns(2017-12-12) Addis, Colin Robert; Keitt, Timothy H.; Daniels, Michael; Di Fiore, Anthony; Jha, Shalene; Meyers, LaurenForaging animals must move to search their environment for food. The given pattern of resources on the landscape, in space and time, shapes the resulting movement patterns of foragers: over evolutionary time, search strategies adapt to the environment, but foragers also respond to short-term changes in the resource distribution resulting from, for example, phenology or plant demographics. Foragers can also alter the landscape as they move, either physically or by facilitating gene flow, such that patterns of alteration in the landscape reflect the movement patterns of foragers. This two-way interaction between movement and landscape patterns sets up a feedback loop: the movement of dispersers alters the landscape upon which they likely base their subsequent movement decisions. In this dissertation, we investigate three aspects of this cyclical interaction in two different systems: crop-pollinating solitary bees in agroecosystems; and frugivorous, seed-dispersing primates in neotropical forests. First, we add phenological variation to an existing, spatially explicit simulation model of pollination services on agricultural landscapes in order to examine how mass-flowering events affect plant-pollinator interactions. Simulation results show that mass-flowering phenology benefits both crop pollination and specialist pollinators, but only when crop phenology and pollinator life-cycles are well synchronized. Second, we examine whether spider monkeys use topography in choosing their foraging routes between fruiting resources by comparing the power of two different movement models—a pure random walk and a model based on elevation—to explain observed foraging movements of spider monkeys in the western Amazon. Our elevation-based model explains the data significantly better than the null model, demonstrating that when given a choice between different routes, monkeys prefer higher-elevation pathways. Finally, in order to understand how spider monkeys shape forest structure through seed dispersal, we present a spatially explicit, agent-based simulation model that links foraging, seed dispersal, and forest growth in a closed cycle. Our results suggest that dispersal acts to condense tree distributions over the long-term, but at intermediate time-scales, it produces network-like landscape patterns that coincide with the spider monkeys’ travel routes.Item Characteristics of ventilation in Callinectes sapidus (Rathbun)(1977) Batterton, Carol Sue Vaughan; Not available1. Ventilation volume (V̇ [subscript G]) was monitored in the blue crab Callinectes sapidus using an electromagnetic flow probe. Mean resting value of ventilation volume for a 200g crab was 146±81ml•min⁻¹, oxygen consumption (V̇ [subscript O₂] was 85.6±35ml • kg⁻¹• hr⁻¹ and extraction efficiency was 48%±14. 2. The relationships between pulsatile ventilatory flow and pressure and pressure phasing in both gill chambers, pressure volume relationships, and ventilatory energy cost were investigated. 3. Blue crabs tolerated exposure to air for at least eight hours at room temperature, and were able to maintain oxygen consumption during exposure to air. 4. Response to 1% CO₂ was variable and inconsistent in S.W. (sea water) Callinectes sapidus. No response to 1% or 2% CO₂ was seen in F.W. (fresh water) blue crabs. 5. Blue crabs responded to hypoxia of 65, 40, and 25 torr by increasing V̇ [subscript G] and maintaining extraction efficiency at the pre-hypoxia value. V̇ [subscript O₂] decreased with decreasing O₂ tension of the water. 6. No significant change was seen in ventilatory parameters in response to salinity variationsItem The coexistence of ecologically similar species(2013-12) Smith, Geneviève Kathleen; Leibold, Mathew A.The biological diversity on planet Earth is astounding. Understanding the origins of this diversity, and how it is maintained, are the twin goals of ecology and evolutionary biology. An early and oft-repeated insight in this investigation is that that similar organisms cannot coexist indefinitely. Theory predicts that individuals and species will compete for limited resources and whichever has even a slight advantage will drive all others extinct in a process known as ‘competitive exclusion’. By diversifying, species avoid competition, thereby ‘stabilizing’ their coexistence. Yet natural systems often display levels of diversity that are surprisingly high, given this theory and investigations of how the similarity of coexisting species is maintained have received much less attention. Using a combination of field studies and experiments I demonstrate that highly similar species of freshwater amphipods may compete for resources without resulting in competitive exclusion. These findings suggest that there exist a range of interactions among Hyalella amphipods, ranging from strong stabilizing effects due to ecological trade-offs, to weakly stabilizing effects, to a total lack of stabilizing effects among various pairs of species in this system. These findings demonstrate how the relative strength of stabilizing forces may vary among coexisting species. Although much effort has been dedicated to enumerating and classifying the ways in which ecological and evolutionary forces promote diversity among species, there has been far less attention paid to mechanisms such as convergent evolution, habitat filtering, competition for non-substitutable resources, and non-ecological speciation, among others. I surveyed current theory that may explain the high levels of similarity among species often found in natural systems. I describe how several ecological and evolutionary mechanisms may operate to promote the coexistence of similar species and present results from new theoretical combinations of mechanisms to demonstrate how they may further act in concert with one another.